1. Field of the Invention
The present invention relates to semiconductor techniques, and more particularly to a semiconductor device and its manufacturing method.
2. Description of the Related Art
With the development of semiconductor techniques, phase-change random access memory (PCRAM) is widely recognized as one of the most promising candidates for memory. As a new and developing non-volatile computer memory technique, PCRAM stores data according to the fact that phase-change materials have different electric resistivities in different states. More particularly, phase-change materials have high resistivities in an amorphous state, and, on the contrary, have low resistivities in a crystalline state. Therefore, data can be stored through phase-change materials switching between amorphous and crystalline states, and then data can be read out by measuring the resistances of phase-change materials.
FIG. 1 schematically shows a structural diagram of a memory cell in a phase-change random access memory.
A phase-change material 130 and 140 are sandwiched between an upper electrode 160 in an insulating material layer 150 and a bottom electrode 120 in an insulating material layer 110. Currently, one kind of phase-change materials commonly used is chalcogenide, such as Ge2Sb2Te5. In a phase-change memory cell having the above structure, joule heat induced by a current flowing between upper electrode 160 and bottom electrode 120 may change the crystal state of a chalcogenide in the phase-change material. Consequently, an amorphous region 140 is generated as the chalcogenide, closer to the bottom electrode 120, changes from crystalline state into the amorphous state.
Below, an existing method of forming phase-change random access memory will be described with reference to FIG. 2A to FIG. 2F.
As shown in FIG. 2A, a first insulating material layer 210 and a second insulating material layer 220 are sequentially formed on a substrate 200, wherein a contact plug 230 is formed in the first insulating material layer 210. A portion of the second insulating material layer 220 is etched off to expose part of the upper surface of the first insulating material layer 210. A bottom electrode material is deposited to form a bottom electrode material layer 240. FIG. 2B is a top view corresponding to FIG. 2A, wherein FIG. 2A is a sectional view taken along a line “A-A” in FIG. 2B.
As shown in FIG. 2C, the bottom electrode material layer 240 is etched to form a step-shaped bottom electrode material layer 250 which is patterned. The bottom electrode material layer 250 has an upper horizontal portion 260, a vertical portion 262, and a lower horizontal portion 264, wherein the lower horizontal portion 264 is in contact with the contact plug 230 in the first insulating material layer 210. FIG. 2D is a top view corresponding to FIG. 2C, wherein FIG. 2C is a sectional view taken along a line “B-B” in FIG. 2D.
As shown in FIG. 2E, an insulating material layer 270 is deposited through a high aspect ratio process (HARP) to cover the bottom electrode material layer 250.
As shown in FIG. 2F, a chemical mechanical process (CMP) is performed, until the upper horizontal portion 260 of the bottom electrode material layer 250 is removed.
Following the above steps, a phase-change material is formed above the vertical portion of the bottom electrode material layer.
According to the above scheme, during the chemical mechanical process in which the upper horizontal portion 260 of the bottom electrode material layer 250 is removed, according to the stacking order of each layer, the insulating material layer 270 and the upper horizontal portion 260 of the bottom electrode material layer 250 are removed in sequence.
When the bottom electrode material layer 250 and the insulating material layer 270 are polished at approximately the same rate, polishing time is commonly used as a CMP stop condition. However, the portion of the insulating material layer 270 to be removed is relatively thick and the bottom electrode material layer 250 is relatively thin, furthermore, the insulating material layer 270 maybe uneven in its thickness. As a result, it is difficult to accurately control the polishing time to stop polishing process immediately after removing the upper horizontal portion 260 of the bottom electrode material layer 250. Therefore, after removing the upper horizontal portion 260 of the bottom electrode material layer 250, the vertical portion 262 of the bottom electrode material layer 250 is likely to be over removed, leading to the unnecessary loss of the bottom electrode.